In Firebird 2 and up, no two BLOB filters in a database may have the same combination of input and output type. Declaring a filter with an already existing
input-output type combination will fail. Restoring pre-2.0 databases that contain such “duplicate” filters will also fail.

The possibility to indicate the BLOB types with their mnemonics instead of numbers was added in Firebird 2. The binary mnemonic for subtype 0 was also added in Firebird 2. The predefined mnemonics are case-insensitive.

User-defined mnemonics: If you want to define mnemonics for your own BLOB subtypes, you can add them to the RDB$TYPES system table as shown below. Once committed, the mnemonics can be used in subsequent filter declarations.